The present invention relates to a semiconductor memory device such as an EPROM (which is an abbreviation of an erasable and programmable read-only memory) device in which information can be erased by ultraviolet light and reprogrammed bit line by bit line with appropriate voltage pulses.
In general, a package for accommodating a chip on which a semiconductor device, such as an EPROM device, is formed with a window for receiving ultraviolet light. When it is necessary to erase the information stored in the EPROM, the entire chip is exposed to strong utlraviolet light which penetrates through the window into the EPROM.
Usually the EPROM is contained in a box to prevent any kind of light from penetrating thereinto. In addition, even during a no erasing mode, the EPROM can be used in a lighted room, since the intensity of the ultraviolet component of the room light is so small that the information stored in the EPROM is not erased.
However, as mentioned above, when the EPROM is in a lighted room, the light penetrates to the p-n junction portions formed by a substrate of one conductivity type and an impurity diffusion region of another conductivity type which serves as a source or a drain of a MIS (Metal-Insulator-Semiconductor) transistor, or a bit line. As a result, junction leakage current is generated therebetween so that the charge of the source or drain of the MIS transistor, or of the bit line is decreased. As a result, the potential of a sense line connected to a sense circuit is also decreased, so that the operation of the device deteriorates.